html {
    height: 100%;
}
body {
    overflow-x: hidden;
    width: 100%;
    height: 100%;
    font-size: 16px;
}
* {
    margin: 0;
    padding: 0;
}
#button {
    position: absolute;
    border-bottom: none;
    background: skyblue;
}
#button button {
    height: 100%;
    width: 33.333%;
    display: inline-block;
}
#button button:last-child {
    margin-right: 0;
}
#div {
    position: absolute;
    border: 1px solid gray;
    cursor: move;
}
canvas {
    position: relative;
    left: 0;
    top: 0;
    margin-left: 2px;
    margin-top: 2px;
    z-index: -2;
}
#div img {
    position: absolute;
    display: inline-block;
    width: 100%;;
    height: 100%;
    left: 0;
    top:0;
    margin-left: 2px;
    margin-top: 2px;
    z-index: -3;
}
.off {
    background: #d5d5d5;
}